I thought back then they had a shorter life expectancy than we do now.
IMHO, it was all a matter of rendering honor to age. It was the usual method at that time to honor the ancient ones with an excessive number of years. Considering that the average longevity, only 2000 years ago, was about 40, it's only obvious that four or six thousand years ago, one could not live that long of a life.
According to Numbers 14:34, the Israelites were doomed to wander in the desert for 40 years just to eliminate all that generation that had left Egypt. It means that at that time, a generation was within the averange of 40 years. The average generation prior to the Flood could have been of even less than that.
Let us take Abraham and Sarah for instance; it was said of Sarah that she was barren, hence she could not bear children. The text says that, at the time Abraham had a vision about the birth of Isaac, they were: Abraham 100 years old and Sarah 90. (Gen. 17:17) A 90 years old woman could not be said that she was unable to bear children because she was barren but because she was too old. Leaving aside the tradition to honor age by adding years to one's age, Sarah could have been on her late 40's and Abraham on his late 50's. Besides, bareness is, in most cases, a temporary condition in a woman prior to menopause. To bear a child, a woman must ovulate, and women do not ovulate after menopause, which occurs between 45 and 55. That's what it means when Sarah laughed at the idea to bear a child.
Talking about a miracle, God would have gone against His own laws, which Baruch de Spinoza says, it would be tantamount to degrade God down to the level of a Greek god. He meant weakness in God.
I am reminded of my late Yemenite wife who was of such a traditional family that I could never find out how old her parents were. Whenever I asked, the answer would always be: "Ad 120 bezrat haShem" which means, till 120 with the help of God. She died at 106, so I was told, although she looked 76.
Well, the bottom line is that, to add years to the years of one's life was a traditional method to honor age. Therefore, the ancient ones from before the Flood might not have been that old after all.